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XMM-Newton Serendipitous Source Catalogue:
3XMM-DR4

The 3XMM-DR4 catalogue is the sixth publicly released XMM-Newton
X-ray source
catalogue produced by the XMM Survey Science Centre (SSC) consortium on
behalf of ESA. It follows the 1XMM (released in April 2003), 2XMMp (July
2006), 2XMM (August 2007), 2XMMi (August 2008) and 2XMMi-DR3 (April 2010)
catalogues: 2XMMp was a preliminary version of 2XMM. 2XMMi and 2XMMi-DR3
were incremental versions of the 2XMM catalogue.

The motivations for creating 3XMM-DR4 are threefold:

1 - A substantial (~50%) increase in the number of public
observations that can be included

2 - Significant improvements to the XMM-Newton Science
Analysis Software and developments with the calibration

3 - The opportunity to advance the scientific quality of the
catalogue through enhancements to the processing algorithms. The key
science-driven gains include:

iv) Extraction of spectra and timeseries for fainter sources,
with improved signal-to-noise

Taken together, these elements yield a catalogue that is about 50%
larger than its predecessor, 2XMMi-DR3, and is, overall, of better
quality and accuracy.

By number of sources, 3XMM-DR4 is the largest X-ray source catalogue
ever produced. 3XMM-DR4 complements deeper Chandra and XMM-Newton small area
surveys, probing a much larger sky area.

The catalogue provides an effective dataset for generating large,
well-defined samples of various types of astrophysical object, notably active
galaxies (AGN), clusters of galaxies, interacting compact binaries and active
stellar coronae, using the power of X-ray selection. The large sky area
covered by the serendipitous survey also means that 3XMM-DR4 is a rich
resource for exploring the variety of the X-ray source populations and
identifying rare source types.

The catalogue contains source detections drawn from 7427 XMM-Newton EPIC
observations made between 2000 February 3 and 2012 December 8; all
datasets were publicly available by 2012 December 31 but not all public
observations are included in this catalogue. The catalogue contains 531261
X-ray detections which relate to 372728 unique X-ray sources (covering
an energy interval from 0.2 keV to 12 keV). About a third of the
observations have features that may cause spurious detections (mainly the
wings of bright sources and large extended emission), and it is strongly
recommended to use a filter (either per source, based on the summary flag
column, or per observation, based on the observation class column).
The following table gives an overview of the statistics of the catalogue
in comparison with the 2XMMi-DR3 catalogue.

3XMM-DR4

2XMMi-DR3

Increment

Number of observations

7427

4953

2474

Number of 'clean' observations (i.e., observation
class < 3)

4553

2789

1764

Observing interval

03-Feb-00 -- 08-Dec-12

03-Feb-00 -- 08-Oct-09

3.2 years

Sky coverage, taking overlaps into account ( ≥ 1ksec exposure)

794 sq.deg

504 sq.deg

290 sq.deg

Number of detections

531261

353191

178070

Number of 'clean' detections (i.e., summary
flag < 3)

432321

284270

148051

Number of unique sources

372728

262902

109826

Number of 'cleanest' (summary flag = 0, not in high-background fields) extended detections

7698

4775

2923

Number of detections with spectra

123867

56017

67850

Number of detections with timeseries

123860

56017

67843

Number of detections where probability of timeseries being constant is < 1 × 10-5

4612

3177

1435

The median flux in the total photon energy band
(0.2 - 12 keV) of the catalogue detections is
~ 2.4 × 10-14
erg cm-2 s-1; in the soft energy band
(0.2 - 2 keV) the median flux is
~ 5.7 × 10-15, and in the hard band
(2 - 12 keV) it is
~ 1.3 × 10-14. About 20% of the sources have
total fluxes below 1 × 10-14
erg cm-2 s-1.

The CSV file has nulls represented by two successive commas, compatible
with Postgres and (probably) other DBMS.

The slimline version of the catalogue contains one row
per unique source (while the the main catalogue has one row per detection)
and thus has 372728 rows. There are 44 columns, essentially those containing information about the unique
sources. The catalogue also contains a column with links to the LEDAS summary
pages. In the case of sources with multiple detections the summary page of
the best detection is selected (i.e., the detection with the
largest exposure time, summed over all cameras), and the summary page gives
cross-links to the other detections.

A separate file is also provided (table below) which contains key
details about the observations used in the construction of the 3XMM-DR4
catalogue. See the
list of observations
for a description of the columns in the file.

Watchouts

1) Shortly after release, an error was noted with the vignetting values in
the 3XMM-DR4 catalogue (and the OMSRLI and OBSMLI pipeline product source
lists), arising from a bug in the
emldetect SAS task. For a given detection (row) in the
catalogue, all values of the vignetting for a given instrument
(i.e. columns <instrument>_<band>_VIG where <instrument>
is PN, M1 or
M2 and <band> is the energy band (1 to 5)) are identical and
equal to the vignetting value at the source position, but for an energy
of 0 keV rather than the intended mid-band energy. It should be
emphasised that this error in the vignetting values does not affect the
count-rates (or fluxes) - these are computed using the exposure maps in
which the vignetting corrections, based on the correct band energies, are
embedded. The error only affects the quoted vignetting columns in the
catalogue and the corresponding entries in the OMSRLI and OBSMLI pipeline
product files

A version of the catalogue with correct vignetting columns will be made
available shortly. Corrected versions of the OMSRLI and OBSMLI source
list files will also be generated and re-ingested in to the XSA in due
course.

2) It has been established that the ODFs of a subset of the mosaic-mode
observations have been incorrectly split in to sub-pointings. Eight
parent observations (involving 63 sub-pointings and 3943 detections in the
3XMM-DR4 catalogue) are known to show clear
temporal overlaps of data between sub-pointings, where there should be no
such overlap. In these cases all sub-pointings and instruments are believed
to be affected. A further ten parent cases (involving 26 sub-pointings, of
which 19 (involving 975 detections) are in the 3XMM-DR4 catalogue) are
known in which some of the
sub-pointings are affected and in these, only MOS1 and/or MOS2 data are
impacted. The cause appears to be related to incorrect time-tagging of
events due to telemetry gaps, which the splitting process handled
incorrectly.

These 18 mosaic-mode parent problem-case observations (encompassing 89
sub-pointings) are listed here.
Two comma-separated-variable format files, one for the cases where
all data are
affected and one for cases where
some data are affected,
are provided which contain the DETID, OBS_ID, RA, DEC and INSTRUMENT(s)
affected for detections from the relevant problematic sub-pointings.

In the cases where all data are affected, this problem has
most obviously manifested itself in (visibly) erroneous timeseries for detections from
the affected sub-pointings. However, it is important to emphasize that
in these cases all the data/products are unreliable. For those cases where
only some
of the data are affected, the SOC believe that only timeseries data/products
are affected and only for the MOS1 and/or MOS2 instruments.

When a full analysis of the problem is completed, the SOC and SSC will
consider what corrective action may be applied in terms of the processed
data, the catalogue and the associated data products. In the meantime,
users are advised to disregard all data/products of detections from the
observations where all data are affected (and any unique source data that
involve them). Users should also disregard timeseries from those cases where
some of the data are affected, though it is advisable to treat other
products/data from these cases with caution too.

3) Whilst the 3XMM-DR4 catalogue values have been shown to be robust, due to an uninitialised variable in a subroutine of the source detection chain, a subset of the error values in the current version of the 3XMM-DR4 catalogue have been over-estimated, and a very small portion of these errors have been under-estimated. Only when a quantity is based on an error-weighted combination of values, such as the EPIC (EP) columns, are the non-error values affected. The quality of the catalogue is therefore assured, but the overall accuracy is in general better than that which is given in the current version.

The error values affected are for the counts, the rate, the flux, the hardness ratio and the position. The worst affected column is the error on the EPIC band 9 flux (EP_9_FLUX_ERR) where 10% of the errors are erroneous, some (~0.1%) by more than a factor 1000. Other columns are in general much less affected, with less than 2% of the errors showing erroneous values in many cases. The RA/dec errors (RADEC_ERR) are erroneous for about 1.4% of detections, with 80% of those values deviating by < ± 0.1 arcsecs. Work is being carried out to rectify the problem. However, until a new version can be produced, the columns affected by this bug are:

Users working on individual sources can re-run the source detection on their fields of interest with SAS 14 tasks which will be released in November 2014.

Nomenclature and Citation

With the new, 3XMM-DR4 catalogue, IAU name nomenclature includes the
designator, 3XMM. The IAU name is provided in the catalogue. The correct,
full nomenclature for a specific detection from the catalogues is the IAU
name, followed by a colon and the detection identification number, that
is:"3XMM Jhhmmss.sSddmmss:detid".

Credits

The production of the 3XMM-DR4 catalogue is a collaborative project
involving the whole XMM-Newton SSC Consortium: